1. Field of the Invention
The present invention relates to a method of forming the end of a connector ferrule fixed around an optical fiber into a convex spherical surface and improves the performance of a connector through reduction in reflected return beams and connection losses which is obtained by performing polishing so that no step is formed between the end faces of the optical fibers and ferrules and by polishing the end faces of the optical fibers with ultra-high precision.
2. Description of the Related Art
In general, an optical fiber is connected to another by securely bonding the optical fibers around the axes of ceramic round bars called ferrules and by aligning the ferrules in a cylindrical sleeve in a face-to-face relationship. Stable performance is obtained by polishing the end faces of the ferrules into convex spherical surfaces with the optical fibers on the vertices thereof to allow the opposite fibers to be brought into abutment by only a small external urging force with little gap (1 .mu.m or less) therebetween.
During the polishing of the end face of a ferrule as described above, the polishing progresses differently on the fiber and ferrule because of the difference in hardness between them since the ferrule surrounding the optical fiber is made of ceramic (alumina or zirconia). Specifically, the polishing progresses faster in the area of the fiber, resulting in a recess in the center of the end of the ferrule. The advent of optical systems of larger capacities and higher speeds has resulted in a need for limiting such a recess to a smaller size. Recently, a polishing technique for obtaining a spherical surface has become essential wherein such a recess is strictly limited to 0.05 .mu.m or less. FIG. 1 shows a conventional method of polishing the end of a ferrule on an optical connector wherein polishing and formation have been carried out by using a resin film 11 tensioned at predetermined force as an abrasive sheet, pressing the end of an optical connector ferrule 8 against the surface of this abrasive sheet, and sliding the end of the optical connector ferrule relative to the film surface with particles 12 (called slurry) of diamond, SiO.sub.2 or the like supplied onto the surface of the abrasive sheet as an abrasive. The abrasive sheet and abrasive agent have been used in the following three types of combination.
The first combination is to use a cellulose type resin film as an abrasive sheet and SiO.sub.2 type colloidal particles including particles of different sizes mixed with water as an abrasive. In this case, the ends of the ceramic ferrule and the fiber are polished taking advantage of a difference in polishing capability between particles which easily become colloidal and particles which do not. This allows a polishing process wherein the size of the step formed between the ferrule and the optical fiber is limited to 0.05 .mu.m or less.
The second combination is a combination of an abrasive sheet having diamond particles applied and fixed thereon and an abrasive comprises of SiO.sub.2 type particles. The third combination is a combination of an abrasive sheet having alumina particles applied and fixed thereon and water as an abrasive.
However, the above-described polishing method has the following problems. Specifically, the first combination has a problem in that a cellulose type abrasive sheet is adversely affected by an alkaline colloidal abrasive constituted by SiO.sub.2 type colloidal particles and alcohol used as a cleaner and is susceptible to chemical deterioration and deterioration of mechanical strength such as deformation of the sheet due to its water-absorbing property. Further, in order to maintain sufficient polishing performance using a SiO.sub.2 solution, uniform mixing must be carried out by means of agitation of abrasive particles using an ultrasonic cleaner or the like. This has resulted in a problem in that many limitations are placed on the usage and maintenance of this combination.
These factors cause significant fluctuation in polishing performance and result in retraction or projection of a fiber from the end face of a ceramic ferrule. This has created a problem in that the step formed by the fiber can not be regulated to 0.05 .mu.m or less and it is therefore difficult to provide a connector having stable connection performance.
The second combination has a problem in that since diamond is hard and sharp enough, even if it is particulate, to produce scratches on the end face of a fiber which form many abrasion-affected layers thereon, the amount of reflected return beams is increased by the fiber and the affected layers which have different refractive indices and the performance of the connector is thus reduced. Further, the abrasive sheet itself is expensive.
The third combination results in a recess at the fiber portion of 0.1 .mu.m or more which does not meet the requirement.
As described above, according to the conventional polishing method using the first, second, or third combination, distortion can be left on the surface of an optical fiber after it is processed and a large step can be formed on the end of an optical fiber ferrule because of the retraction or projection of the fiber. Therefore, such a method can not be regarded as a technique for polishing an optical connector ferrule with stable and ultra-high quality at a low cost.
The present invention has been conceived taking the above-described situation into consideration, and it is an object of the invention to provide a method of polishing a high performance optical connector ferrule which allows the end of an optical connector ferrule to be polished with stable and high quality at a low cost by eliminating both the generation of abrasion-affected layers on the surface of the optical fiber and the formation of a step between the surface of the ferrule and the fiber.